In the present specification, reference is made to the following publications cited for illustrating prior art techniques, in particular with regard to optically non-linear components and applications thereof in ultrafast laser techniques.    [1] L. Brzozowski et al. in “IEEE J. Quantum Electron. 36(5), 550-555 (2000);    [2] J. Leuthold et al. in “Nature Photon.” 4, 535-544 (2010);    [3] Q. Lin et al. in “Opt. Express” 15, 16604-16644 (2007);    [4] L. Waldecker et al. in “Nat. Mater. Nature Publishing Group” (2015), NMAT4359;    [5] V. Grigoriev et al. in “New Journal of Physics” 12, 053041 1367-2630/10/053041 (2010);    [6] N. Moshonas et al. in “Proc. of SPIE” Vol. 9131 913129-1 (2014);    [7] N. Kaiser and H. K. Pulker “Optical interference coatings” Springer, 2013, A. V. Tikhonravov, chapter “Design of Optical Coatings”, pp. 81-104;    [8] A. V. Tikhonravov et al. in “Appl. Opt.” 35, 5493-5508 (1996); and    [9] A. V. Tikhonravov et al. in “Appl. Opt.” 51, 7319-7332 (2012).
It is generally known that optical multilayer coatings, e.g., allowing precise control of group delay dispersion (GDD) characteristics over more than one octave or providing high-damage-threshold broadband characteristics are incorporated into the majority of ultrafast lasers, in particular femtosecond lasers. Ultrafast lasers have enabled unprecedented quality in micro-machining applications, have considerably increased medical capabilities and have brought imaging techniques like multiphoton fluorescence microscopy and optical coherence tomography to a new level. Furthermore, ultrafast lasers are unique tools in research applications in fields like attosecond physics, frequency comb metrology and femtochemistry.
Current femtosecond laser oscillators yield average output powers of several hundreds of W and pulse energies of several tens of μJ at repetition rates of more than 10 MHz. These laser parameters correspond to peak powers >50 MW, thus outperforming even some laser-amplifiers. At such intense optical fields, non-linearities in interference coatings may be revealed and exploited. Various non-linear components are used in laser physics, which can be divided into two general classes: firstly, components based on non-linear absorption with finite relaxation time, and secondly, components based on instantaneously (<5 fs time scale) occurring non-linear response.
The first class includes components comprising a stack of metallodielectric layers (MDS) and numerous kinds of (real) saturable absorbers. MDS components have a significant non-linear optical response so that they are suitable in particular for providing non-linear mirrors having an amplitude dependent reflectance. However, applications of these mirrors are restricted to the visible spectral range, which is outside the range of interest of multiple ultrafast laser applications. Furthermore, MDS components may have disadvantages in terms of a step-like dependency of the output intensity on the input and a slow recovery after irradiation on a time scale of about 20 ps.
Semiconductor saturable absorber mirrors (SESAM) of the first class have been widely used for mode-locking in laser oscillators, however, finite relaxation time prevents the generation of pulses shorter than 700 fs in Yb:YAG lasers. Moreover, they are strongly wavelength dependent, difficult to grow and characterize, introduce non-saturable losses, reveal two-photon absorption (TPA) processes, have rather moderate modulation depth for high power operation (less than 2%), have low damage threshold and are quite expensive.
The second class includes artificial saturable absorber components and components based on the instantaneous Kerr effect. Artificial saturable absorber components include e.g., single-mode fibers exploiting non-linear phase shifts for additive-pulse mode-locking, or components using Kerr lensing in a gain medium. Due to broad bandwidth, these techniques have advantages over SESAMs for generating shorter pulses. Nevertheless, they impose serious constraints on the cavity design and alignment sensitivity.
The nearly instantaneous nature of the electronic non-linear response (due to Kerr effect and TPA) of silicon leads to many applications related to high-speed optical-signal processing [2, 3]. As a further matter, a recent study [4] has shown that optical modifications of a phase-change material Ge2Sb2Te5 (GST) are one order of magnitude larger than those achievable with silicon and present a new route to high-speed optical modulators for communications and computations. Furthermore, alternating layers of materials possessing opposite Kerr non-linearities have been modelled and analytically analyzed [5]. Dielectric quasicrystals have been theoretically studied and found to demonstrate an interplay between the intrinsic spatial asymmetry of the structure and Kerr non-linearity causing bistability and multistability of transmission (whereas the reflection has not been studied) sensitive to the propagation direction [5]. Besides, the presented components require an auxiliary pump signal in order to have no limitations of the maximum value of transmission.
Another optical component of the second class, comprised of successive thin-film layers possessing high values of non-linear susceptibility, has been numerically investigated [6]. Specifically, optical bistability which translates to a change in the value of reflectivity with the increasing input power might have been observed. The authors of [6] considered doped polymer films PPAA (push-pull aromatic azobenzene) possessing a non-linear Kerr refractive index as high as about 1.7×10−6 cm2/W, which was plugged into the calculations as −1.7×10−6 cm2/W and most probably have led to an improper result. The behavior of another Bragg structure comprised by 30 pairs of linear layers of transparent ceramic Sc2O3 and layers of antiferromagnetic Co3O4 having a non-linear refractive index of 1×10−6 cm2/W at 405 nm has been numerically studied in [6] as well. Moreover, the authors of [6] predicted a hysteresis phenomenon when strong non-linearity is present.
The structure described in [6] has not been put into practice, probably due to simulating the non-linear response of exotic materials, whose non-linear refractive indices are indeed rather high, but unrealistic for practical applications. Moreover, Moshonas et al. [6] have oversimplified the situation in the calculations: The authors have not considered possible issues related to nonlinear absorption processes, arising together with the desired change of refractive index induced by the external optical irradiation.